Scratching the Surface

Listen up, chucklehead! The joke here is that someone is talking to a detective about a crime, maybe even a murder. They point out that "he" scratched his face, implying that's what did him in. Then comes the punchline: the person casually asks, "Did I do that?" It's funny because they're suggesting they might be the culprit, but they act all innocent and clueless. Classic case of dark humor, see?